Undesirable stresses

Basically, with the higher pressures it is possible to develop tighter dimensional tolerances with higher mechanical performance, but there is also a tendency to develop undesirable stresses (orientations) if the processes... 

In some cases with TPs it is possible to insert metal fittings, such as bushes, into the molded component, using friction or ultrasonic methods of insertion, and fusing the TP around the metal insert. With RTP compounds, this may create undesirable stresses, however, and advice should be sought from the material supplier. 

The material is now in the cavity of the mold. It has reached the cavity in a warpage flowable state and must now solidify under the influence of the relatively low mold temperature. This will allow the finished molded part to be removed. The solidification of the molded part must occur under controlled conditions so that no undesirable stresses develop within the molded part. These stresses would warp the molded part. 

Peel stresses exist at the ends of adhesively bonded lap joints, especially in single-lap joints. To reduce these undesirable stresses the use of mechanical fasteners near the joint ends is sometimes recommended. As high peel stresses occur only in the immediate vicinity of the joint ends, the fasteners have to be located very close to the joint ends and cannot provide a fail-safe load path in the case of debonding. Therefore, the use of mechanical fasteners to reduce peel stresses is not recommended here. Instead the bonded joint should be correctly designed to reduce the peel stresses to an acceptable level. 

Although adhesives offer a number of advantages, they suffer from a number of limitations, listed in Table V, which must also be considered when assessing the suitability of an adhesive for a particular application. Most structural adhesives are strong in shear and tensile loading, but weak when peel or cleavage stresses are present. Joint design is therefore necessary to eliminate, as far as possible, these undesirable stresses. 

Five main factors contributing to undesirable stress at work have been identified by Karasek (see Further Reading at the end of this chapter). 

In workplaces such as offshore gas and oil rigs, and at remote rrrinesites, or in fact ary worksite where there is an on-site food and drink facility or canteen, nutrition becomes an aspect of concern, becanse it has a bearing on cancer arrd on other diseases, inclnding those emerging above, such as diseases of the heart and arteries (cardiovascitlar disease - CVD). Undesirable stress arrd smoking also contribute toCVD. 

Supports. The spaciag of supports is governed by the hot allowable stress of the piping materials stabiUty, ia the case of large-diameter thin-wall pipe deflection to avoid sagging or pocketing and the natural frequency of the unsupported length to avoid susceptibiUty to undesirable vibration. 

Materials of Construction. GeneraHy, carbon steel is satisfactory as a material of construction when handling propylene, chlorine, HCl, and chlorinated hydrocarbons at low temperatures (below 100°C) in the absence of water. Nickel-based aHoys are chiefly used in the reaction area where resistance to chlorine and HCl at elevated temperatures is required (39). Elastomer-lined equipment, usuaHy PTFE or Kynar, is typicaHy used when water and HCl or chlorine are present together, such as adsorption of HCl in water, since corrosion of most metals is excessive. Stainless steels are to be avoided in locations exposed to inorganic chlorides, as stainless steels can be subject to chloride stress-corrosion cracking. Contact with aluminum should be avoided under aH circumstances because of potential undesirable reactivity problems. 

In the last chapter we said that one of the requirements of a high-temperature material - in a turbine blade, or a super-heater tube, for example - was that it should resist attack by gases at high temperatures and, in particular, that it should resist oxidation. Turbine blades do oxidise in service, and react with H2S, SO2 and other combustion products. Excessive attack of this sort is obviously undesirable in such a highly stressed component. Which materials best resist oxidation, and how can the resistance to gas attack be improved ... 

The resulting mouldings and extrudates are consequently anisotropic and mouldings can be four to five times as strong in one direction as in another direction. This can lead to planes of weakness and easy fracture when subject to shock (impact) stresses. Generally such orientation is undesirable but there are at least two instances of its being of value ... 

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

Another observation on this solution is that if the Poisson s ratios of the fiber and the matrix are not the same (they are likely different), then longitudinal stresses are induced in the fiber and matrix (with a net resultant longitudinal force of zero) with accompanying shearing stresses at the fiber-matrix boundary. Such shearing stresses will naturally arise under some stress states. Thus, this material characteristic cannot be regarded as undesirable or indicative of an inappropriate solution. 

Adsorbers, distillation colunuis, and packed lowers are more complicated vessels and as a result, the potential exists for more serious hazards. These vessels are subject to tlie same potential haz. uds discussed previously in relation to leaks, corrosion, and stress. However, llicse separation columns contain a wide variety of internals or separation devices. Adsorbers or strippers usually contain packing, packing supports, liquid distributors, hold-down plates, and weirs. Depending on tlie physical and chemical properties of the fluids being passed tlirough tlie tower, potential liazards may result if incompatible materials are used for llie internals. Reactivity with llie metals used may cause undesirable reactions, which may lead to elevated temperatures and pressures and, ullinialely, to vessel rupture. Distillation columns may contain internals such as sieve trays, bubble caps, and valve plates, wliicli are also in conlacl with tlie... 

It is possible with solvents of a particular composition to determine quantitatively the level of stress existing in certain TP products where undesirable or limited fabricated-in stresses exist. The stresses can be residual (internal) stresses resulting from the molding, extrusion, or other process that was used to fabricate the plastic product. Stresses can also be applied such as bending the product. As it has been done for over a half century, the... 

The recommended radius not only reduces the brittleness effect but also provides a streamlined flow path for the plastic melt in the mold cavity. The radiused corner of the metal in the mold reduces the possibility of its breakdown and thus eliminates a potential repair need. Too large a radius is also undesirable because it wastes material, may cause sink marks, and may even contribute to stresses from having excessive variations in thickness. 

Oil-canning The property of a panel that flexes past a theoretical equilibrium point, and then returns to the original position. This motion is analogous to the bottom of a metal oilcan when pressed and released. Part flexing can cause stress, fracturing, or undesirable melting of thin-sectioned, flat parts. 

Moisture can also result in undesirable dimensional changes occurring in AN—K nitrate based expls, caused by changing crystal habit on thermal cycling. The physical stresses produced may cause stress failures of their containers (Refs 26 32)... 

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